The methods for coating a carrier are roughly classified into the following two kinds.
One of the methods is called a solvent-coating method such as a fluidized layer type spray-coating method in which a coating solution is prepared by dissolving a coating resin in a solvent and the resulting coating solution is spray-coated on the surfaces of magnetic particles by making use of a fluidized layer and is then dried up; a dip-coating method in which a coating solution is prepared by dissolving a coating resin in a solvent and magnetic particles are dipped in the resulting coating solution to be coated thereon and then dried them up; and a sintering type coating method in which a coating solution is prepared by dissolving a coating resin in a solvent and is then coated on the surfaces of magnetic particles and, further, the resin is sintered; and
the other of the two methods is called a dry type coating method such as those methods described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP OPI Publication) Nos. 2-8860/1990 and 3-144579/1991, in which a mechanical impact is applied to a resin to be coated on and fixed to a carrier.
An improvement of the environmental difference in the chargeability of a developer has been attempted by making a carrier hydrophobic, as described in JP OPI Publication No. 59-228261/1984 on a silicon resin-coated carrier).
The advantages of a dry-type coating carrier are that a carrier can be prevented from becoming large in particle-size because no solvent is used therein, so that a granularity can be made lower, and that a yield can be increased, because any required particle-size distribution can be provided. Further, because no drying step is not required, so that a carrier can be prepared in a short time and the productivity can also be higher. In addition to the above, a coating treatment can be so performed as to make the coated surfaces uniform without any unevenness as compared to any solvent-coating type carriers, because a resin is fixed by applying a mechanical impact after fine-particles are made uniformly adhered in advance by making a stirring treatment in the first processing step.
However, when coating two or more kinds of fine-particles, each kind of the particles can hardly be mixed up together so that the fluidity of a coating agent may seriously lowered, and a great deal of coated places and a few coated places are localized. Therefore, any uniformly coated surfaces cannot be obtained. In particular, such a phenomenon as mentioned above is liable to produce when the chargeability and/or particle-sizes of individual fine-particles are different from each other. A carrier having a unevenly coated layer is slow in charging rise-time so that a toner-flying phenomenon is liable to produce especially under a low-temperature and low-humidity conditions, because the mixability thereof with toner is low.
When making use of a means for making carriers hydrophobic, an environmental difference is little in the case where the toner side is relatively hydrophobic. However, as in recent years, when a toner is added thereto with inorganic fine-particles for improving the characteristics thereof, the toner is liable to adsorb moisture by itself and the triboelectric series thereof is varied by an environment. Therefore, rather on the contrary, when making a toner hydrophobic, the environmental difference of the charged amounts of a developer is resulted to be greater. When the environmental difference of the charged amounts thereof is greater, the charging amount is lowered under a high-temperature and high-humidity conditions and, resultingly, an image-transfer property us lowered, because an image-transfer failure and so forth are produced. Under the low-temperature and low-humidity conditions, a charged amount is made higher, so that a lack of developability may resultingly be produced and so it goes. Therefore, when an environment is changed, such an inconvenience may be produced that the resulting images are seriously varied, even if the same developer should be used.